Calculating Transformer size for upgrading.

[EdCampbell]

Hi,

Can anyone out there help me. We are in the process of upgrading the transformer that feeds our plant & I have the awesome task of having to calculate more or less what is the right size. I have a list of all my present and future loads loads which are a mixture of 3, single and bi phase. Ok what I did was this. Added up all the single phase (110 v) loads, then I added the 220 bi phase loads. I then divided this by three to give me a more or less 3 phase load. Then I added this figure to the 3 phase loads and then calculated my full load current. Just to give me a bit of a real idea I then took the final figure and multiplied it by 80 %.

I calculated a transformer of about 300 kva other people who are I may contract to do the work have calculated about 225 kva.

Is the method correct? (I need an answer quite urgently.) I am based in Mexico.

If you want I can send you my list of loads they are in Kw and hp.

Please can you contact me directly at

(Moderator’s Note: Edited to remove email address. If you wish to send an email message to this person, then first send a Private Message via this Forum, and ask for the email address.)

Thanks for your help,

Ed Campbell

 

[jtester]

The easiest way is to work in KW or KVA. You just add up the loads, single phase120+biphase+3phase and them convert to amps by dividing by voltage x 1.732.

If you aren't governed by the NEC, you'll find this method sometimes produces too large a total load since it assumes that all loads are on simultaneously. There may be room for some diversity.

Jim T